1. Field of the Invention
This invention relates to compensation for the proximity effect in the imaging patterns of electron beam projection lithography and more particularly to mask structures and a method of manufacturing such masks.
2. Description of Related Art
Terminology
To avoid ambiguity or confusion, some of the terms used in the present application are defined in the following definitions. The definitions are made as clear as possible by limiting them to simple electron beam (E-beam) lithography of integrated circuits, but analogous definitions apply to applications other than the patterning of integrated silicon circuits and to other generalizations. The specificity of these definitions is not intended to limit the generality of the invention.
Electron Beam Projection Lithography: An E-beam is directed through a mask towards an E-beam lens where it is focused onto a work piece such as a semiconductor wafer creating an image of the mask pattern on the wafer. PA0 Transparent: Electrons pass through the material with little or no scattering. PA0 Nontransparent: Most electrons are either absorbed in the material, scattered elastically (with little loss of energy) through large angles or scattered inelastically (with significant loss of energy) through smaller angles. The net result of these interactions and lenses and a contrast aperture downstream of the mask is that few electrons strike the target or workpiece. PA0 Projection Mask (Reticle): A master pattern of transparent and nontransparent areas which determines that electrons hitting transparent areas will strike the target and that electrons hitting nontransparent areas will not strike the target. Usually the master pattern is projected onto the target at a significantly reduced size as the E-beam is focused to expose a smaller image area. PA0 Scattering Mask: A projection mask in which the nontransparent areas generally scatter the electrons somewhat (add relatively small random deviations to their original vectors) instead of absorbing or reflecting them. PA0 Scattering-mask Lithographic Projection System: An electron beam system incorporating, in sequence, an electron source, a scattering mask, a back focal plane where electrons spreading directly from the source are to be focused, and a target plane where electrons spreading directly from the scattering mask are focused. Such a system requires, as a minimum, one lens (generally in the vicinity of the scattering mask) to focus the source at the back focal plane, and one lens (generally in the vicinity of the back focal plane) to focus the mask at the target. PA0 Back Focal Plane Filter: A mask of transparent and nontransparent areas. The filter is placed at a plane where the original electron source is imaged. Such a filter can be designed to pass, scatter or block electrons striking it in different places. PA0 Target: A silicon wafer coated with a "resist" material that becomes more easily removed from certain areas of the wafer when it is developed (dissolved) after exposure of those areas to an electron beam. Thus, after chemical processing the resist material is entirely removed from all areas of the target where the exposure by the projected electron beam was sufficiently intense. Because of uncontrollable variations in the "chemical processing" faithful reproduction of the intended pattern is possible only when there is a significant difference between minimum exposure in areas intended to be exposed and the maximum exposure in areas intended to remain unexposed. PA0 Backscattering: Electrons impact upon the top surface of a silicon target covered with resist at various points of incidence on that surface. The electrons are scattered within the silicon of the target so that they re-emerge from the surface of the silicon and produce additional resist exposure at a significant distance from the original impact point. The accelerating voltage applied to the electrons before they strike the target has a significant effect on the backscatter distribution profile, which may extend only a few micrometers or tens of micrometers from the impact point. PA0 Proximity Effect: A pattern-dependent variation in general exposure levels. Electrons pass through a layer of resist on the top surface of a silicon substrate target striking the underlying silicon substrate. Some of those electrons are scattered back upward, producing additional exposure of the resist by the electrons on their second pass. Because the backscattered electrons typically spread over an area much larger than a minimum-size pattern feature, an isolated spot may get only 5% or less additional exposure; however, accumulated backscattering from many adjacent spots can increase effective resist exposure by 80% or more. PA0 Blockout regions: Large 2 mm square transparent areas of the mask which result in 500 microns square (0.5 mn square) exposed features on the silicon wafer after 4:1 reduction, leaving a large opening with all resist removed. PA0 Clean (Clear): Refers to exposed areas on the wafer from which all resist is removed by the development process. PA0 Subresolution Scattering Features (SSF): Reticle features smaller than the smallest features which can be resolved at the target. A uniform distribution of such features on the reticle has the net effect of reducing the exposure dose in the corresponding area at the target.
Using those nominal percentages and assuming that direct exposure is 100 units, it is seen that large exposure areas receive 180 units, small unexposed areas in the middle of such large exposed areas receive 80 units, and small isolated exposed areas receive 105 units of exposure. That 105-to-80 difference is not a comfortable margin for process control. The proximity effect causes dense pattern areas to be overexposed and/or sparse pattern areas to be underexposed.
U.S. Pat. No. 4,712,013 of Nishimura et al for "Method of Forming a Fine Pattern with Charged Particle Beam" describes a method for forming fine patterns having dual exposure. In a first exposure an exposed pattern is formed with a charged beam E-Beam (Electron-beam) exposure. The second (auxiliary) exposure floods the entire device in a non-pattern exposure with either a charged beam or an electromagnetic wave selected from an E-beam, an ultraviolet (UV) or far UV ray or an X-ray. The auxiliary exposure can be performed at a high voltage such as a voltage of more than 30 KeV.
U.S. Pat. No. 4,504,558 of Bohlen et al for "Method of Compensation of Proximity Effect in Electron Beam Projection Systems" describes compensation of proximity effects by following a first E-beam exposure step with a second exposure step to expose selected areas in the second exposure step. The proximity effect is used with complementary masks. The developing process of the photoresist is discontinued prematurely. Asymmetrical ridge edges are detected by viewing them under a microscope. The line width of the narrower ridge is measured, thereby obtaining the proximity effect range.
Japanese abstract No. JP56-165325 (A) of Nishizawa describes formation of a pattern using sequential exposure levels by shifting the photomask relative to the workpiece coated with photoresist. Each exposure is made with energy less than "proper", but the overlap provides a level of exposure which is proper and when the resist is developed the process yields the selected pattern.
Ismail "A Novel Method for Submicron Structurization Using Optical Projection Lithography" in Microelectronic Engineering 1 (1983) pp 295-300 describes an optical projection method for exposing a very small feature by a multiple (double) exposure when exposing the regions of an unexposed resist layer to be patterned during each exposure with a level of optical energy below the energy level needed to expose the whole resist thickness.
The above references illustrate a problem which requires a solution, which is that multiple exposure steps have been required to achieve the desired results. It is an object of this invention to achieve the objectives achieved by the above processes with a new process employing a single exposure step.
U.S. patent application Ser. No. 355,887, now U.S. Pat. No. 5,532,496 of Gaston for "Proximity Effect Compensation in Scattering-Mask Lithographic Projection Systems and Apparatus Therefore" describes an alternative single step method that uses the electrons scattered outside of the normal acceptance angle of the simple contrast aperture of a scattering mask E-beam projection lithography system. Several different kinds of back focal plane filters allow one to tailor the amount of scattered electron signal that is passed to the target. However, the proximity correction cannot be optimized on an individual feature by feature basis. The mesh size over which the correction must be averaged out is equal to the exposure subfield size. Since throughput considerations will keep this subfield size reasonably large, significant compromises in proximity correction optimization must be made. As contrasted with Gaston, the present invention decouples the proximity mesh size and the exposure subfield size, permitting feature by feature optimization of the proximity corrections.
U.S. Pat. No. 5,256,505 of Chen et al for "Lithographical Mask for Controlling the Dimensions of Resist Patterns" relates to a lithography mask with intensity modulation lines with dimensions below the resolution capability of the tool. The lines have the effect of reducing exposure in the center of large features. Since the lines have dimensions (sizes) below the resolution capability of the tool they are not seen in the final image.
U.S. Pat. No. 4,902,899 of Lin et al for "Lithographic Process Having Improved Image Quality" relates to an improved image quality by having a plurality of elements in the mask which have dimensions below the resolution capability of the tool. They are employed to control the transmittance area of the mask.
U.S. Pat. No. 5,256,505 of Chen et al and U.S. Pat. No. 4,902,899 of Lin et al are very specifically addressed to optical lithography problems. The only mention of particle beam lithography is a very general statement at Col. 4 (lines 47-51) of Chen et al. Sub-resolution shapes are implemented here to address nominal energy differences received by specific shapes in optical lithography. Those references are different from the present invention which combats the classic E-beam proximity effect which is based on the neighborhood of the exposed feature and intensity variations due to scattered electrons as is explained below.